Robber fly
Location:  Fairfield, Maine
August 3, 2010 9:40 pm
Dear Bugman,
I was taking photos of this Robber fly on my hammock stand, when it swooped at my face and caught a flesh fly (I think) right in front of me and landed back where I was photographing it to eat. I have checked BugGuide and believe this is female Efferia pogonias. Does it have a common name or nickname? Do you know what the little honey-colored sacs on its sides are? There is one between the wing and the behind the wing and the ”shoulder” of the hind-most leg. You can also see them looking down through the wings from the top.
James R

Robber Fly eats fly

Hi James,
We cannot tell if the prey in your photo is a Flesh Fly, but it is definitely another Dipteran.  You may be correct that this is
Efferia pogonias based on images posted to BugGuide, but we believe, based on the shape of the tip of the abdomen, that this is a male.  Perhaps someone with more experience can confirm.  The vestigial wings you asked about are known as halteres, and they are primarily for balance according to the Orkin Fly Anatomy web page.  According to BugGuide:  “haltere noun, plural halteres. – two small knobbed appendages rising from each side of the thorax in the order Diptera just where the posterior pair of wings would arise were they present, and to which they are analogous. They tend to balance the insect in flight.

Hi Daniel,
Thanks for the I.D. and extra info!
I never would have guessed those were vestigial wings;  What a cool feature.
Best regards,

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